Allowed U.S. patent application Ser. No. 08/784,873, filed Jan. 16, 1997 in the name of Hatzipapafotiou, Ser. No. 08/786,599, filed Jan. 21, 1997 in the name of Schiavoni, and U.S. patent application Ser No. 08/916,600, filed Aug. 22, 1997 in the name of Hudson, describe spacecraft- or satellite-based communication systems (the ACES system, for example) which are compatible with GSM-style land-based cellular systems, but which solve problems which are unique to spacecraft-based systems. The GSM system is described in detail in the text The GSM System for Mobile Communications, subtitled A Comprehensive Overview of the European Digital Cellular System, authored by Michel Mouly and Marie-Bernadette Pautet, and published in 1992 by the authors, at 4, rue Elisee Reclus, F-91120 Palaiseau, France. Another text describing the GSM system is Mobile Radio Communications, by Raymond Steele, published 1992 by Pentech Press, London, ISBN 0-7273-1406-8. Spacecraft-based systems such as ACES provide communications between terrestrial terminals or stations, some of which are principally mobile user terminals, and others of which are fixed gateway terminals. The gateway terminals provide connections between the spacecraft-based communication systems and land-based public switched telephone systems or public land mobile networks. The necessity for a plurality of such gateway terminals in a large-scale ACES-type system arises from the fact that it may not be economically feasible to interconnect all the land-based networks as would be required if only one gateway terminal were provided in the communication system; for example, if the eastern portion of the Asian continent is to be provided with service, it may be cheaper to provide a gateway terminal on each of the major islands, such as Sumatra, Borneo, Luzon, and Mindanao, than to provide an underwater cable which would connect the land systems of the various islands to the continent's land-based system.
In the system as described in the abovementioned Schiavoni and Hatzipapafotiou patent applications, mobile subscribers always lie within a spot beam of the spacecraft antenna, and can always be accessed for communication with any of the gateways. The ACES system provides service principally between user terminals and gateways, although it also provides for user-terminal-to-user-terminal communication under some conditions. Improved spacecraft-based communication systems are desired.